1. Field of the Invention
The present invention relates to a solid-state imaging device, a manufacturing method thereof, and an imaging apparatus using the solid-state imaging device.
2. Description of the Related Art
As a new structure of a solid-state imaging device, JP-A-2003-031785, for example, proposes a so-called back-illuminated image sensor that receives an incident light from a surface which is opposite to a surface on which pixel transistors, a peripheral circuit portion, and wiring layers are formed.
This structure eliminates a vignetting phenomenon of the incident light due to the wiring layers and thus can improve quantum efficiency of the incident light.
On the other hand, an image sensor is known having a structure such that a light receiving portion receives an incident light from a side thereof on which wiring layers are formed. However, such an image sensor has a limitation on applicable materials since it is necessary to secure a sufficient transmissivity at a desired wavelength range so that a loss of the incident light above a light incidence surface of the light receiving portion can be suppressed.
On the contrary, the back-illuminated image sensor has a structure such that, when the image sensor is actually operated, the layer that forms the wiring region is positioned on the back surface (rear surface) side relative to the light receiving portion. Therefore, the back-illuminated image sensor has an advantage of completely eliminating the limitation on applicable materials.
As a technique of a substrate for the back-illuminated image sensor, SOI (silicon on insulator) substrates are popular.
A specific example of the SOI substrate has a configuration that a single-crystalline silicon layer having a thickness of several μm and serving as an active layer is formed on a base substrate via a silicon oxide layer. The silicon oxide layer provides selectivity when the base substrate is removed at the time of forming the back-illuminated image sensor.
However, it is known that, since the image sensor is an analog device, imaging characteristics thereof are strongly influenced by unevenness in processing dimension of a film thickness or line width, PID, crystal quality, and other factors.
Among these factors, the request for reduced noise components is particularly strong. Thus, suppression of metal contamination during wafer processing steps as well as securement of high-quality crystals are very important requirements.
However, the SOI substrate has a structure such that the active layer is formed by the single-crystalline silicon layer having a thickness of several μm as described above. Therefore, a defect-free zone, that is, a DZ (denuded zone) layer, which contributes to photoelectric conversion, occupies most areas of the active layer, and thus there is not enough area for a gettering layer.
Therefore, it is very difficult to suppress noise components due to metal contamination, which can be said to be the most important problem of an image sensor.
Next, a specific example of a gettering technique will be described.
In general, gettering sites are formed in a region away from an active layer in which devices are formed, in order to prevent characteristic deterioration. Specifically, gettering sites are formed inside a silicon (Si) substrate at a distance of several tens of μm from a back surface of the silicon substrate or an active layer.
Another requirement is that contaminated seeds are caused to reach gettering sites and be trapped there by a diffusion phenomenon which is expected to occur at the time of heat treatment during the manufacturing process.
Moreover, depending on a method used, it may be indispensable to consider a phenomenon that the requirements (contaminated seeds and crystal defects) determining the formation of gettering sites are extended (redistributed) by the heat treatment.
The SOI substrate is applicable to the image sensor having the known structure under the above-described limitations. Thus, it is considered difficult to apply a plurality of gettering techniques.
As illustrated in FIG. 16, a gettering technique is generally classified into an intrinsic gettering (IG) method and an extrinsic gettering (EG) method.
The intrinsic gettering method is a method in which oxygen existing in an oversaturated condition (1×1018 cm−3) in a silicon (Si) substrate is precipitated to form gettering sinks, such as SiO2 precipitates, dislocations, or stacking faults, only inside the wafer. The oxygen is diffused towards the outer side of the wafer surface to form a defect-free zone (DZ layer), and devices are formed in the defect-free zone.
The extrinsic gettering method includes a phosphorous gettering method that uses diffusion of phosphorous (P). This method uses misfit dislocations, which are formed by diffusing phosphorous (P) impurities to high concentration, as the gettering sinks. The extrinsic gettering method also includes a method of forming a heterogeneous film. This method uses stress-induced strains of a polysilicon or silicon nitride (Si3N4) film, which is different from a silicon (Si) substrate, as the gettering sinks.
The extrinsic gettering method also includes a method using ion implantation or laser irradiation. This method uses cracks, dislocations, or stacking faults, which are formed due to damage created by ion implantation or laser irradiation, as the gettering sinks. A so-called carbon gettering is one example of this method.
The extrinsic gettering method also includes a hydrochloric acid (HCl) gettering method. This method subjects a wafer to heat treatment in a gaseous atmosphere containing chlorine (Cl) to change heavy metal into volatile metal chlorides, thus removing the volatile metal chlorides from the wafer.
The phosphorous gettering method has several problems in terms of the ability to control with high accuracy the misfit dislocations to be formed in a region away from the active layer and the redistribution of the contaminated seeds during the heat treatment to be trapped properly. However, at the present moment of time, it is considered difficult to apply the phosphorous gettering method to gettering of the substrate for solid-state imaging devices.
Moreover, the HCl gettering method is considered difficult to apply to gettering of the substrate for solid-state imaging devices since its gettering effect is transient. Furthermore, the ion implantation/laser irradiation method is considered difficult to apply to gettering of the substrate for solid-state imaging devices since there is basically potential concern about the method redistributing contaminated materials.
On the other hand, as a carbon gettering method which exhibits excellent gettering ability for metallic contaminated seeds, JP-A-2005-294705, for example, proposes a method of selectively forming gettering sites. However, as illustrated in FIG. 17, this method may not eliminate the effect of carbons (C) that are diffused and redistributed by heat treatment after the gettering sites of carbons (C) are formed, which makes miniaturization difficult.
In addition, in the SOI substrate, a silicon oxide film formed by a thermal oxidation method is present between the silicon layer of the active layer and the base substrate. Due to presence of the silicon oxide film, when gettering sites are formed on the side of the base substrate, there is such a limitation that diffusion or trapping (gettering) of metallic contaminated seeds other than metal elements such as copper capable of diffusing in the silicon oxide film are inhibited by the silicon oxide film.
As described above, a gettering technique, which is effective for solid-state imaging devices (e.g., back-illuminated image sensors) using the SOI substrate, is not yet established at the present moment of time.